Dual mode electromechanical variable speed transmission apparatus and method of control

ABSTRACT

A dual-mode electro-mechanical variable speed transmission includes an input shaft, an output shaft system, a gear system having at least three branches, two electric machines, and at least a clutch. The first electric machine couples to a branch of the gear system, the output shaft system couples to another branch of the gear system, the input shaft couples to the remaining branch or one of the remaining branches of the gear system, and the second electric machine selectively couples either to the same branch that is coupled to the output shaft system with a speed ratio or to one of the remaining branches that is not coupled to the first electric machine with a different speed ratio. The transmission provides at least two power splitting modes to cover different speed ratio regimes. The transmission can also provide at least a fixed output shaft to input shaft speed ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. Ser. No. 12/744,584 filed May25, 2010 as a National Phase Application of PCT InternationalApplication No. PCT/CN2008/001945, International Filing Date Nov. 28,2008, claiming priority of Chinese Patent Application 200710195199.8,filed Dec. 4, 2007, which are incorporated by reference herein.

FIELD OF THE INVENTION

The invention is related to a dual mode electro-mechanical variablespeed transmission. It can be used in a wide variety of vehicleapplications and power equipment.

BACKGROUND OF THE INVENTION

An internal combustion engine operates in a certain range of speed andpower. Inside this range, there usually exists a smaller regime wherethe engine achieves optimal performance. On the other hand, drivingconditions vary enormously, not only in wheel speed but also in drivingtorque at the drive wheels. A combination of a given speed and torquedefines a power state. Matching the power state of the engine with thatof the drive wheels is one of the primary functions for a transmission.

In recent years, the development of hybrid technology provides newavenues for achieving improved matching in power state between theinternal combustion engine and the drive Wheels. Among variouspower-train architecture designs, the most representative is theelectro-mechanical continuous variable transmission, known as ToyotaHybrid System, or THS. THS uses the power split principle, splitting theinput power into two paths of different type. Part of the input powerpasses through a mechanical power path which is comprised of gears andshafts; the rest passes through an electric power path which containselectric machines. The device that splits the power is a simpleplanetary gear system. THS has only one power splitting mode andprovides a single output to input speed ratio node point SR where allpower passes through the mechanical power path. When the transmissionoperates at a speed ratio higher than the speed ratio node point,internal power circulation occurs. One of the power paths sees morepower than the power transmitted through the transmission, which reducespower transmission efficiency. This, to a large extent, constrains theeffective operating speed ratio of the transmission. For high powervehicle applications, the power ratings for the electric machines haveto be increased significantly.

SUMMARY OF INVENTION

One of the objectives of the current invention is to overcome theaforementioned drawbacks of prior art by providing a novel family ofdual-mode electro-mechanical variable speed transmissions that share thecommon design feature and performance characteristics. Thesetransmissions can operate under at least two different power splittingmodes, offering higher power transmission efficiency by avoidinginternal power circulation. These transmissions are capable of providingcontinuously variable output shaft to input shaft speed ratio andindependent power regulation in a wide range. They significantly extendthe operational speed ratio range.

To achieve above objectives, the current invention provides thefollowing technical solution:

A dual-mode electro-mechanical variable speed transmission includes agear system, an input shaft, an output shaft system, at least oneclutch, and two electric machines along with their electric drives andcontrollers. Said gear system can be a three-branch planetary system,having at least three co-axial rotatable members or components. Each andevery rotatable component constitutes a branch in the gear system. Thethree-branch gear system is also known as the three-shaft gear system.It has two degrees of freedom: the speeds of any two branches uniquelydetermine the speed of the remaining branch. Using vertical vectors torepresent the rotational speeds of the three branches of the gear systemand lining the vectors up in the horizontal direction with predetermineddistances between them, a so-called speed nomograph, shown in FIG. 1, isconstructed. Each vector corresponds to a branch. Each branch accordingto its position from left to right (or from right to left) on the speednomograph, is referred to as the first (I), second (II) and third (III)branch of the gear system, respectively. The end point of speed vectorslies on a straight line, which is referred to as the speed line. For athree-branch gear system, the relationship among the rotational speedsof the branches of the gear systems is uniquely defined by acharacteristic parameter of the gear system K_(a), where K_(a) is thedistance between the first branch and the second branch, assuming thedistance between the second and third branch is 1.0 unit. The unit canbe inch or centimeter, or any engineering measures in length. The gearsystem is connected to the input shaft, the output shaft system, and tothe first and second electric machines in the following configuration:the input shaft connects to a branch of the gear system, establishing afixed speed ratio between them; the output shaft system connects toanother branch of the gear system, establishing a fixed speed ratio; thefirst electric machine connects to the remaining branch of the gearsystem, establishing a fixed speed ratio; the second electric machineselectively connects to the two branches of the gear system that are notin directly connection with the first electric machine, establishing atleast two speed ratios, respectively.

The gear system can also be a four-branch gear system having at leastfour co-axially rotatable members or components. Each of the co-axialrotatable components is associated with a corresponding branch of thefour-branch gear system. The four-branch gear system is also known asfour-shaft gear system. The four-branch gear system also has two degreesof rotational freedom. That is to say, the speeds of any two branches ofthe four-branch gear system uniquely determine the speeds of remainingbranches. A four-branch gear system can be represented by a four-branchspeed nomograph, as shown in FIG. 2. The vertical vectors in FIG. 2represent the rotational speeds of the four branches. Similarly,according to their positions from left to right (or from right to left)on the four-branch speed nomograph, these four branches are referred toas the first branch (I), the second branch (II), the third branch (Ill)and the fourth branch (IV), respectively. The distances between thespeed vectors which are associated with the corresponding branches ofthe four-branch gear system, are uniquely determined by thecharacteristic parameters K_(a) and K_(b) of the gear system. Here it isassumed that the distance between the fourth and the third branches is1.0 unit. K_(a) is defined as the distance between the third branch andthe second branch; K_(b) is defined as the distance between the thirdbranch and the first branch. The four-branch gear system is connected tothe input shaft, the output shaft system and the first and secondelectric machines in the following configuration: the input shaftconnects to one of the four branches with a fixed speed ratio; theoutput shaft system connects to another branch of the four-branch gearsystem with a fixed speed ratio; the first electric machine connects toone of the two remaining branches of the four-branch gear system with afixed speed ratio; the second electric machine selectively connectseither to the same branch that is connected to the output shaft systemwith a speed ratio or to the last remaining branch of the four-branchgear system with a different speed ratio.

The dual-mode electro-mechanical variable speed transmission includes atleast a counter shaft and at least a clutch installed on the countershaft. The second electric machine selectively couples to the twodifferent branches of the gear system via the counter shaft and theclutch or clutches, providing two different power splitting modes. Toensure speed synchronization between the components that the clutch isto connect during the shifting between the two power split modes, thespeed ratios between the second electric machine and the branch itconnects and between the second electric machine and the branch it isabout to connect must satisfy a predetermined proportional relationship.Said relationship is determined by the characteristic parameters of thegear system. At the switching point between the different powersplitting modes, the torque of the second electric machine isessentially zero. No torque impact exerts on the clutch. This leads to asmooth, continuous and non-interruptive operation in terms of speed,torque and power for components associated with the input shaft, theoutput shaft system and the first and second electric machines.

Said dual-mode electro-mechanical variable speed transmission mayfurther include a brake or brakes. In general, the clutch and the brakeare referred to as torque transfer devices. Through a coordinatedoperation of said torque transfer devices, the transmission may functionas a stepwise transmission, providing at least a fixed speed ratio, inaddition to the continuously variable speed ratio. Fixed speed ratiooperation may be desirable for special applications.

The current invention also provides a method for design, producing andoperating said dual-mode electro-mechanical variable speed transmission.Said method includes following steps: (1) Produce a planetary gearsystem; said planetary gear system includes at least four co-axialrotate-able components with each corresponding to a branch of the gearsystem; said planetary gear system has two degrees of rotationalfreedom; the speeds of any two branches uniquely determine the speeds ofall other branches in the planetary gear system; said planetary gearsystem can be represented by a speed nomograph; the distance between thefirst and third branches is denoted by K_(b), the distance between thesecond and third branches is K_(a), the distance between the fourth andthe third branches is 1 unit. (2) Provide a first electric machine and asecond electric machine; the maximum continuous power ratings of theelectric machines are set to be no less than P_(em). (3) Design andproduce an input shaft, making it capable of transmitting a maximumpower no less than P_(in). (4) Design and produce an output shaftsystem. (5) Design and produce a clutch or clutches, having at least anengagement position. (6) Connect, with a fixed speed ratio, the firstelectric machine to the first branch of the planetary gear system;connect, with a fixed speed ratio, the output shaft system to the secondbranch of the planetary gear system; connect, with a fixed speed ratio,the input shaft to the third branch of the planetary gear system, andselectively connect the second electric machine to the second branch ofthe planetary gear system with a speed ratio of GR×GR1 or to the fourthbranch of the planetary gear system with a speed ratio of GR×GR2, whereGR×GR1 is the speed ratio of the second branch to the second electricmachine, and GR×GR2 is the speed ratio of the fourth branch to thesecond electric machine; in doing so, a dual-mode variable speedtransmission is constructed. (7) Operate said transmission to provide atleast two different operation modes; switching between operating modesis achieved by using clutch or clutches; at switching point, thecomponents to be engaged by the clutch or clutches are automaticallysynchronized with no discontinuity in speed; at switching point, thespeed ratio between the second branch and the third branch of theplanetary gear system is denoted by SR_(b). (8) Select thecharacteristic parameters K_(a) and of the planetary gear system suchthat the following relationship holds true,

${{\frac{\left( {K_{a} + 1} \right)\left( {1 - {SR}_{b}} \right)}{K_{a} \cdot {SR}_{b}} + 1} = \frac{{GR}\; 2}{{GR}\; 1}};$$\frac{K_{b}\left( {K_{a} + 1} \right)}{K_{b} - K_{a}} \leq \left( \frac{1 + {P_{em}/P_{i\; n}}}{1 - {P_{em}/P_{i\; n}}} \right)^{2}$

Above mentioned technical solution has following benefits: it offers anovel dual-mode electro-mechanical variable speed transmission withreduced power demands on electric machines. Said transmission has simpleand compact structure and low manufacturing cost. It is capable ofproviding continuous operation from reverse to stop to forward, withoutrequiring the conventional launching device. Said transmissionsignificantly improves the overall efficiency of the vehicle.

BRIEF DESCRIPTION OF THE DRAWING

In the accompany drawings which form part of the specification:

FIG. 1 is a three-branch speed nomograph, describing the rotationalspeed relationship among co-axial rotating components of thethree-branch gear system;

FIG. 2 is a four-branch speed nomograph, describing the rotational speedrelationship among co-axial rotating components of the four-branch gearsystem;

FIG. 3 is a schematic diagram of a preferred embodiment (embodiment 1)of dual-mode electro-mechanical transmission of the current invention;

FIG. 4 is a schematic diagram of another preferred embodiment(embodiment 2) of dual-mode electro-mechanical transmission of thecurrent invention;

FIG. 5 is a schematic diagram of another preferred embodiment(embodiment 2A) of dual-mode electro-mechanical transmission of thecurrent invention;

FIG. 6 is a schematic diagram of another preferred embodiment(embodiment 2B) of dual-mode electro-mechanical transmission of thecurrent invention;

FIG. 7 is a schematic diagram of another preferred embodiment(embodiment 3) of dual-mode electro-mechanical transmission of thecurrent invention;

FIG. 8 is a schematic diagram of another preferred embodiment(embodiment 3A) of dual-mode electro-mechanical transmission of thecurrent invention;

FIG. 9 is a schematic diagram of another preferred embodiment(embodiment 4) of dual-mode electro-mechanical transmission of thecurrent invention;

FIG. 10 is a schematic diagram of another preferred embodiment(embodiment 4A) of dual-mode electro-mechanical transmission of thecurrent invention; and

FIG. 11 is a schematic diagram of another preferred embodiment(embodiment 4B) of dual-mode electro-mechanical transmission of thecurrent invention.

DETAILED DESCRIPTION OF THE INVENTION

The current invention can have various embodiments and configurationsthat incarnate the spirit of current invention. Embodiments andconfigurations disclosed thereafter in text and in illustrations areused for the purpose of explanation only and shall not be interpreted aslimitation to the scope of the current invention. The following detaileddescription illustrates the invention by way of example and not by wayof limitation.

FIG. 3 shows a preferred embodiment, the embodiment 1 of currentinvention. The dual-mode electro-mechanical variable speed transmissionis comprised of a three-branch gear system, an input shaft (Input), anoutput shaft system (Output), a first clutch (CL1), a second clutch(CL2), a counter shaft (CTS), a first electric machine (EM1) and asecond electric machine (EM2), along with the associated drives andcontrollers (not shown) for the electric machines. Said three-branchgear system is a simple planetary gear-train, having a ring gear (R), aset of planet gears (P), a planet carrier C and a sun gear (S). The ringgear (R) is in internal meshing engagement with the planet gears (P);the sun gear (S) is in external meshing engagement with the planet gears(P). The sun gear (S) constitutes the first branch (I) of thethree-branch gear system; the planet carrier (C) constitutes the secondbranch (II) of the three-branch gear system; and the ring gear (R)constitutes the third branch (III) of the three-branch gear system. Thecharacteristic parameter K_(a) of the three-branch gear system isuniquely determined by the teeth number Z_(R) of the ring gear and theteeth number of the sun gear Z_(S). The output shaft system furtherincludes a differential (Diff), a first half-shaft (Output_1) and asecond half-shaft (Output_2). Each electric machine is comprised of arotor and a stator. For three-branch gear system shown in embodiment 1,the characteristic parameter is expressed as:

$\begin{matrix}{K_{a} = \frac{Z_{R}}{Z_{S}}} & (1)\end{matrix}$

In embodiment 1, the rotor of the first electric machine EM1 is directlycoupled to sun gear S, establishing a fixed connection with the firstbranch (I) of the three-branch gear system with a fixed speed ratioof 1. The input shaft (Input) couples to the planet carrier C via atorsion damper or a shack load absorber (Dmp), establishing a connectionwith the second branch (II) of the three-branch gear system with a fixedspeed ratio of 1. The output shaft system (Output) couples to ring gearR via two pairs of gears G1A and G1B, and G4A and G4B, establishing aconnection with the third branch (III) of the three-branch gear systemwith a speed ratio of GR1×GR4. The second electric machine EM2, coupleseither to the ring gear R through gears G1A and G1B, and gears G3A andG3B, establishing a first selective connection to the third branch ofthe three-branch gear system with a speed ratio of GR1×GR3, or to theplanet carrier C through gears G2A and G2B, and gears G3A and G3B,establishing a second selective connection to the second branch of thethree-branch gear system with a speed ratio of GR2×GR3. Here GR1, GR2,GR3 and GR4 are gear teeth ratios, defined respectively as,

$\begin{matrix}{{{{GR}\; 1} = \frac{Z_{G\; 1B}}{Z_{G\; 1A}}};{{{GR}\; 2} = \frac{Z_{G\; 2B}}{Z_{G\; 2A}}};{{{GR}\; 3} = \frac{Z_{G\; 3B}}{Z_{G\; 3A}}};{{{GR}\; 4} = \frac{Z_{G\; 4B}}{Z_{G\; 4A}}}} & (2)\end{matrix}$where Z denotes the number of teeth with its subscript representing thecorresponding gear. For example, Z_(G1A) denotes the number of teeth forgear G1A. The structure and connection characteristics that theembodiment 1 represents can be expressed as:S(EM1)-C(Input,EM2[CL2])-R(Output,EM2[CL1])

Each term, separated by the symbol “-” in above expression represents abranch. The total number of terms represents the number of branches ofthe gear system. In this example, the expression has three terms, thusrepresenting a three-branch gear system. The first term represents thefirst branch of the three-branch gear system; the second term representsthe second branch of the three-branch gear system and the third termrepresents the third branch of the three-branch gear system. The symbolused in each term denotes the corresponding rotational components of thegear system. For example, the first term S in above expressionrepresents the sun gear. The content in the round brackets followingeach term represents the component or components that is connected tothe branch the term represents. For example, the first term S(EM1) inabove expression denotes that the first electric machine EM1 isconnected to the sun gear S at the first branch of the gear system. Whenthe connection of a component to a branch is made through a clutch, thecorresponding clutch is placed in square brackets after the component.In this case, the connection is a selective connection. For example, thesecond term C(Input,EM2[CL2]) in above expression denotes that the inputshaft (Input) is connected to the carrier C at the second branch of thegear system and the second electric machine EM2 is selectively connectedto the carrier C through clutch CL2 at the second branch.

The first and second electric machines EM1 and EM2, along with theirrespective drives, are electrically connected to transmit power. Whenthe dual-mode electro-mechanical variable speed transmission is used ina hybrid vehicle, the hybrid system may further include an energystorage device (BT, not shown in illustrations) to store and recaptureenergy.

When the second electric machine EM2 is connected to the ring gear R,clutch CL1 engages and clutch CL2 disengages. The transmission isoperated under an output power split mode. The input power is split intotwo paths and transmitted to the output shaft system. One is themechanical path that goes from the input shaft (Input), through theplanet carrier C, the planets P, the ring gear R and the output gearsets G1A, and G1B, and G4A and G4B to the output shaft system (Output);the other is the electrical path that goes from input shaft (Input),through the planet earner C, the planets P, the sun gear S, the firstelectric machine EM1, the second electric machine EM2, gear set G3B,G3A, the counter shaft CTS, clutch CL1, and gear set G4A and G4B to theoutput shaft system (Output).

When the second electric machine EM2 is connected to the planet carrierC, clutch CL2 engages and clutch CL1 disengages. The transmission isoperated under a so-called input power split mode. Similarly, the inputpower is split and transmitted to the output shaft system through twopower paths. The mechanical power path goes from the input shaft(Input), the planet carrier C, the planet gears P, the ring gear R, andthe gear sets G1A and G1B, and G4A and G4B to the output shaft system(Output); the electrical power path goes from the input shaft (Input),gear set G2A and G2B, clutch CL2, counter shaft CTS, gear set G3A andG3B, the second electric machine EM2, the first electric machine EM1,the sun gear S, and then through the planet carrier C, the ring gear R,gear sets G1A and G1B, and G4A and G4B to the output shaft system(Output).

To facilitate the description, the ratio of the output shaft system(Output) speed (speed of gear G4B) to the input shaft (Input) speed isdefined as the output-to-input speed ratio of the transmission and issimply referred to thereafter as the speed ratio, SR. A speed ratio nodeis defined as a specific value in speed ratio at which at least one ofthe electric machines achieves zero rotational speed. A speed ratio nodeis also referred to as a speed ratio node point.

The first embodiment (embodiment 1) is capable of providing a naturalspeed ratio node SR0 where the speed of the output shaft system is zeroand a regular speed ratio node SR1. At the regular speed ratio node, atleast one of the electric machines achieves zero rotational speed. If atransmission offers inure than one regular speed ratio nodes, inascending order, these regular speed nodes are referred to as the firstspeed ratio node, the second speed ratio node and so on, respectively.For embodiment 1, the regular speed ratio node SR1 is the first speedratio node where the rotational speed of the first electric machine iszero. The natural speed ratio node SR0 divides the entire speed ratioregime into a forward speed ratio regime and a reverse speed ratioregime. Above the natural speed ratio node is the forward regime; belowthe natural speed ratio node is the reverse regime. The first speedratio node SR1 further divides the forward regime into a low speed ratioregime and a high speed ratio regime. Below the first speed ratio nodeis the loss speed ratio regime and above the first speed ratio node isthe high speed ratio regime.

At the first speed ratio node point SR1, the torque of the secondelectric machine EM2 is zero if there is no net power exchange betweenthe transmission and the energy storage device. Thus, it would beadvantageous to choose SR1 as the switching point between differentpower splitting modes to avoid or reduce shock load in torque for thetransmission. In the low speed ratio regime below SR1, the transmissionadopts output power split operation mode; in the high speed ratio regimeabove SR1, the transmission adopts input power split operation mode. Inthe reverse regime, the hybrid system operates under pure electric drivemode. Thus, the power in each path, whether mechanical path or electricpath is always less than the power transmitted through the transmissionfrom the input shaft to the output shaft system. No internal powercirculation exists in any speed ratio regime for the dual modeelectro-mechanical variable speed transmission. The speed ratio range ofthe transmission is thus effectively extended. To ensure speedsynchronization of the clutch at the mode switching point SR1, thefollowing relationship between the gear ratios has to be satisfied,

$\begin{matrix}{\frac{{GR}\; 2}{{GR}\; 1} = \frac{K_{a}}{K_{a} + 1}} & (3)\end{matrix}$

The operation of the dual mode electro-mechanic variable speedtransmission shown in FIG. 3 is described below.

Slow Speed Region.

Before the vehicle starts moving, the transmission operates under lowspeed ratio regime. The second electric machine EM2, couples to theoutput shaft system through clutch CL1. The first electric machine EM1is idling with rotational speed in the same direction as the input shaftthat is coupled to the internal combustion engine. The second electricmachine EM2 is at zero speed. When the vehicle starts, the electricdrive (CTRL, not shown) of the electric machine delivers electric powerto the second electric machine, providing drive torque according tocommends it receives. The drive torque is amplified through two-stagegears G3A and G3B, and G4B and G4A, and applied to the output shaftsystem (Output). At this moment, except for small amount internallosses, the second electric machine EM2 consumes or converts essentiallyno power. The vehicle is at stand still, there is only torque demand andno power demand of the drive wheels. The torque demand is primarilysupplied by the second electric machine EM2. Concurrently, the internalcombustion engine provides no torque for vehicle launching, and thusdelivers essentially zero power to the input shaft. As the secondelectric machine increases its drive torque, the vehicle starts to move.Consequently, the second electric machine starts to rotate. Therotational speed of the first electric machine decreases as the vehiclegradually picking up the speed. Accordingly, the second electric machinestarts to consume electric power, converting it into the requiredmechanical power to drive the vehicle. The consumed electric power isfully or partially supplied by the first electric machine through theelectric drive (CTRL). To balance the torque of the first electricmachine, the internal combustion engine is now providing necessary drivetorque. After vehicle started moving, the drive torque required by thevehicle is provided collaboratively by both the internal combustionengine and the second electric machine EM2. The torque of the secondelectric machine reduces gradually.

As the speed of the vehicle continues to increase, the speed of thesecond electric machine increases and its torque further reduces. Incontrast, the speed of the first electric machine continues to decrease,till it becomes zero. At this point, the first electric machine reachesits zero speed node point. Correspondingly, the transmission arrives atits regular speed ratio node point SR1. Assuming there is no netelectric power exchange between the transmission and the energy storagedevice, the speed ratio of the transmission at which the speed of thefirst electric machine EM1 becomes zero coincides with the speed ratioat which the torque of the second electric machine EM2 is zero.

High Speed Region

The speed ratio node point SR1 of the transmission is the dividing pointbetween the slow and high speed ratio regimes. At this switching pointSR1, the torque of the second machine is zero (assuming there is no netelectric power exchange between the transmission and energy storagedevice); the rotational components associated with clutches CL1, CL2 aresynchronized. During the switching of the operation mode, the secondclutch CL2 engages, connecting the second electric machine EM2 to theinput shaft. Following this event, the first clutch CL1 disengages,releasing the second electric machine EM2 from the output shaft system.In doing so, the transmission adopts the input power split operationmode to avoid internal power circulation.

As the speed of vehicle further increases, the speed ratio of thetransmission increases to exceed the speed ratio node point SR1. Thespeed of the first electric machine EM1 starts to increase from zero ina reversed direction with respect to internal combustion engine. Thespeed of the second electric machine EM2 varies with speed of engine ata fixed speed ratio. Assuming there is no net electric power deliveredto or received from the energy storage device, the torque of the secondelectric machine will be increasing in a reversed direction from zeroconcurrently. Under this condition, the second electric machine EM2 actsas a generator, converting mechanical power into electric power to feedthe first electric machine EM1. The first electric machine EM1 acts as amotor, converting electric power into mechanical drive power.

The transmission shown in FIG. 3 provides at least one fixedoutput-to-input speed ratio of the transmission to fulfill the demandsthat may be required in certain applications. The fixed output-to-inputspeed ratio operation is achieved by engaging both clutch CL1 and clutchCL2. Under this condition, the transmission transmits power throughmechanical power path.

The first embodiment, embodiment 1, has other variations. For example,the connection of the input shaft to the planetary gear and theconnection of the output shaft system to the planetary gear areexchangeable; that is to say, the output shaft system connects to thesecond branch C of the three-branch gear system and the input shaftconnects to the third branch R. This leads to embodiment 1A. Thestructure and connection characteristics of embodiment 1A can beexpressed as:S(EM1)-C(Output,EM2 [CL2])-R(Input,EM2 [CL1])

Accordingly, the condition for synchronization of clutches is set forthby,

$\begin{matrix}{\frac{{GR}\; 2}{{GR}\; 1} = {1 + \frac{1}{K_{a}}}} & (4)\end{matrix}$

FIG. 4 shows embodiment 2 of the current invention. It represents atypical example of the second type of embodiments. The transmissionshown in FIG. 4 is comprised of a four-branch planetary gear system, aninput shaft (Input), a counter shaft (CTS), an output shaft system(Output), a first clutch CL1, a second clutch CL2, a first electricmachine EM1 and a second electric machine EM2 along with the electricdrives (CTRL, not shown) for the electric machines. The four-branchplanetary gear system is constituted by a Ravigneaux planetarygear-train. It contains a large sun gear S1, a small sun gear S2, a setof long planet gears PL, a set of short planet gears PS, a ring gear Rand a planet carrier C. Each of the long planet gears PL is in internalmeshing engagement with the ring gear R and in external meshingengagement with the large sun gear S1; each of the short planet gears PSis in external meshing engagement with a corresponding long planet gearPL and with the small sun gear S2. Said small sun gear S2 constitutesthe first branch (I) of the four-branch gear system, the ring gear Rconstitutes the second branch (II), the planet carrier C constitutes thethird branch (III) and the large sun gear S1 constitutes the fourthbranch (IV). The characteristic parameters K_(a), and K_(b) are uniquelydetermined by the teeth number of the ring gear Z_(R), the teeth numberof the large sun gear Z_(S1), and the teeth number of the small sun gearZ_(S2).

$\begin{matrix}{{K_{a} = \frac{Z_{S\; 1}}{Z_{R}}};{K_{b} = \frac{Z_{S\; 1}}{Z_{S\; 2}}}} & (5)\end{matrix}$

Referring to FIG. 4, the transmission further includes a first set ofmeshing gears G1A and G1B, a second set of meshing gears G2A and G2B, athird set of meshing gears G3A and G3B and a fourth set of meshing gearsG4A and G4B. The output shaft system includes a differential (DM), afirst output half-shaft (Output_1) and a second output half-shaft(Output_2). Each electric machine comprises a rotor and a stator,respectively.

The rotor of the first electric machine EM1 is coupled directly to thesmall sun gear S2, establishing a fixed connection with the first branchof the four-branch gear system with a speed ratio of 1.0. The outputshaft system (Output) couples to the ring gear R via two stages ofmeshing gears G1A, G1B and G4A, G4B, establishing a fixed connection tothe second branch of the four-branch gear system with a speed ratio ofGR1×GR4. The input shaft (Input) couples to the planet carrier C througha torsion damper (Dmp), establishing a connection to the third branch ofthe four-branch gear system with a speed ratio of 1.0. The secondelectric machine EM2 couples selectively either to the ring gear Rthrough two stages of gears G1A and G1B, and G3A and G3B, establishing afirst selective connection to the second branch of the four-branch gearsystem with a first speed ratio of GR1×GR3, or to the large sun gear S1through two stages of gears G2A and G2B, and G3A and G3B, establishing asecond selective connection to the fourth branch of the four-branch gearsystem with a second speed ratio of GR2×GR3. Here GR1, GR2, GR3 and GR4are gear ratios defined previously. The structure and connectioncharacteristics of embodiment 2 can be expressed as:S2(EM1)-R(Output,EM2[CL1])-C(Input)-S1(EM2[CL2])

The notations and definitions of symbols employed in above expressionare the same as previously described.

Similarly, the first and second electric machines along with theirrespective drives and controllers are electrically connected fortransmitting electric power and signals. When used in hybrid vehicleapplications, the system may further include an energy storage device(BT) for storing and recapturing energy.

When the second electric machine EM2 is selectively connected to thering gear R, clutch CL1 engages and clutch CL2 disengages. Thetransmission operates under the so-called output power split mode. Inputpower is transmitted to the output shaft system through two power paths.One is the mechanic power path that goes from the input shaft (Input),through the planet carrier C, the long planet gear PL, the ring gear R,gear sets G1A and G1B, and G4A and G4B to the output shaft system(Output); the other is the electric power path that goes from the inputshaft (Input), through the planet carrier C, the small sun gear S2, thefirst electric machine EM1, the second electric machine EM2, gear setG3B and (33A, clutch CL1, gear set G4A and G4B to the output shaftsystem (Output).

When the second electric machine EM2 is selectively connected to thelarge sun gear S1, clutch CL2 engages and clutch CL1 disengages. Thetransmission operates under a so-called compound power split mode. Inputpower is delivered to the output shaft system through two power paths: amechanical power path and an electric power path. The mechanical powerpath goes from the input shaft (Input), through the planet carrier C,the long planet PL, the ring gear R, gear sets G1A and G1B, and G4A andG4B to the output shaft system (Output); the electric power path goesfrom the input shaft (Input), through the planet carrier C, the largesun gear S1, gear set G2A and G2B, clutch CL2, gear set G3A and G3B, thesecond electric machine EM2, the first electric machine EM1, the smallSun gear S2, then through the large sun gear S1, the ring gear R, gearsets G1A and G1B, and G4A and G4B to the output shaft system (Output).

Embodiment 2 is capable of providing three output-to-input speed rationode points, including a natural speed ratio node point SR0 where theoutput shaft system is at zero speed and two regular speed ratio nodepoints, SR1 and SR2. At a speed ratio node point, at least one of theelectric machines is at zero rotational speed. The natural speed rationode point SR0 divides the entire speed ratio regime into a forwardregime and a reverse regime. Above SR0 is the forward regime and belowSR0 is the reverse regime. The first speed ratio node point SR1, furtherdivides the forward regime into low and high speed ratio regimes. BelowSR1 is the low speed ratio regime, and above SR1 is the high speed ratioregime.

At the first speed ratio node point SR1, the second electric machine EM2provides zero torque if there is no net power exchange between thetransmission and the energy storage device. Therefore, it is beneficialto choose SR1 as the switching point for changing power spilt mode inorder to reduce or avoid possible impact torque load to transmission.Taking into consideration of possible internal power losses of theelectric machines and the associated drives, the actual switching pointin SR may be in the vicinity of SR1. The output power split operatingmode is adopted at the low speed ratio regime below SR1, the compoundpower split operating mode is adopted at the mid to high speed ratioregime above SR1. In reverse regime pure electric drive mode could beadopted to effectively avoid internal power circulation. To ensurerotational speed synchronization for clutch at a chosen switching speedratio SR_(s), the gear ratios have to satisfy following relationship:

$\begin{matrix}{\frac{{GR}\; 2}{{GR}\; 1} = {1 + \frac{\left( {K_{a} + 1} \right)\left( {1 - {{GR}\;{1 \cdot {GR}}\;{4 \cdot {SR}_{s}}}} \right)}{{K_{a} \cdot {GR}}\;{1 \cdot {GR}}\;{4 \cdot {SR}_{s}}}}} & \left( {6a} \right)\end{matrix}$where GR1×GR4×SR_(s) is the speed ratio of the second branch (II) to thethird branch (III) of the four-branch gear system, and is denoted asSR_(b). That is SR_(b)=GR1×GR4×SR_(s). Substituting SR_(b), into theequation (6a), yields

$\begin{matrix}{\frac{{GR}\; 2}{{GR}\; 1} = {1 + \frac{\left( {K_{a} + 1} \right)\left( {1 - {SR}_{b}} \right)}{K_{a}{SR}_{b}}}} & \left( {6b} \right)\end{matrix}$

When choosing the first speed ratio node point SR1 as the operating modeswitching point, that is to say SR_(s)=SR1, above equation can besimplified as

$\begin{matrix}{\frac{{GR}\; 2}{{GR}\; 1} = \frac{K_{b} + 1}{K_{b} - K_{a}}} & \left( {6c} \right)\end{matrix}$

The functions and the operations of the dual-mode electro-mechanicalvariable speed transmission shown in FIG. 4 are described below:

Continuously Variable Speed Operation

1. Low Speed Ratio Regime

Before the vehicle starts to move, transmission is set to operate in lowspeed ratio regime. The second electric machine EM2 couples throughclutch CL1 to the output shaft system (Output). The first electricmachine EM1 is idling and rotating in the direction opposite to that ofthe internal combustion engine. The second electric, machine EM2 is atzero speed. As the vehicle starts, the controller sends commends todrive circuit. The drive circuit, in turn, provides the requiredelectric power to the second electric machine EM2, generating a drivingtorque. The drive torque is amplified through two-stage gear sets G3Aand G3B, and G4B and G4A, and delivered to the output shaft system. Atthis moment, except for an insignificant amount of internal powerlosses, the second electric machine EM2 does not yet convert anyelectric power into mechanical power. Because the vehicle is still at astandstill, there is no requirement for drive power but drive torque atthe drive wheels. The drive torque to move the vehicle comes primarilyfrom the second electric machine EM2. Concurrently, the internalcombustion engine provides zero starting torque for the vehicle, thusthere is no power output form the engine. As the torque of the electricmachine increases, the vehicle accelerates from standstill and movesforward.

Accordingly, the second electric machine EM2 starts to rotate.Meanwhile, the speed of the first electric machine EM1 gradually reducesto comply with the increased vehicle speed. As the second electricmachine EM2 rotates, it begins to consume electric power, converting theelectric power into the required mechanical drive power. The consumedelectric power is fully or partially provided by the first electricmachine EM1 through electric drives and controllers (CTRL). To balancethe torque load of the first electric machine EM1, the internalcombustion engine must provide the necessary torque. After the vehiclestarts moving, the drive torque at the drive wheels is shared betweenthe internal combustion engine and the second electric machine EM2,causing the torque of the second electric machine EM2 to decrease.

As the speed of vehicle increases, the rotational speed of the secondelectric machine EM2 increases but its torque continues to decrease. Onthe contrary, the speed of the first electric machine EM1 continues todecrease until it reaches zero. At the moment when the first electricmachine EM1 stops rotating, the transmission arrives at its firstregular speed ratio node point SR1. Assuming there is no net electricpower exchange between the transmission and the energy storage device,the zero torque point (a speed ratio node point that corresponds to thezero torque of an electric machine) of the second electric machine EM2coincides with the zero speed point (a speed ratio node point thatcorresponds to zero speed of an electric machine) of the first electricmachine EM1.

2. High Speed Ratio Regime

The first speed ratio node point SR1 marks the transition from a lowspeed ratio regime to a high speed ratio regime, or vice versa. At theoperation mode switching point, the second electric machine EM2 is atzero torque and the engaging components of the first and second clutchesare synchronized. At this moment, the second clutch CL2 engages,connecting the second electric machine EM2 to the large sun gear S1through gear sets G3A and G3B, and G2A and G2B. Immediately after thesecond clutch's engagement, the first clutch CL1 starts to disengage,disconnecting the second electric machine EM2 from the output shaftsystem. Transmission is now operating under the compound power splitmode.

As the speed of the vehicle further increases, the speed ratio of thetransmission increases, exceeding the first regular speed ratio nodepoint SR1. The rotational speed of the first electric machine EM1, risesfrom zero and continues to increase in the same direction as the inputshaft from the internal combustion engine. The speed of the secondelectric machine EM2 starts to decrease. At this moment, should there beno net electric power exchange between the transmission and energystorage device, the torque of the second electric machine EM2 will risefrom zero and increase in value acting in the opposite direction. Thesecond electric machine EM2 functions as a generator, providing electricpower to the first electric machine EM1 and/or to the energy storagedevice. The first electric machine EM1 acts as a motor, convertingelectric power into mechanical power.

As the vehicle continues to increase in speed, the speed of the secondelectric machine EM2 continues to decrease until it becomes zero. Thetransmission then reaches its second speed ratio node point SR2. At thisspeed ratio node point, the power transmitted through the electric powerpath becomes zero; all power is transmitted from input shaft to theoutput shaft system through the mechanical power path.

Between the first speed ratio node point SR1 and the second speed rationode point SR2, the power split ratio PR, defined as the powertransmitted through the electric power path to the power delivered atthe input shaft, possesses a local maximum value. The maximum value isdependent upon the characteristic parameters of the four-branch gearsystem. Assuming the maximum input power of the transmission is P_(in)and the maximum continuous power rating of the electric machine isP_(em), the power ratio of the maximum continuous power rating of theelectric machine to the maximum input power of the transmission isdenoted as PR_(max)=P_(em)/P_(in). For an adequate matching between thesizes of electric machines and the construction of the transmission, sothat the transmission can be operated continuously and appropriatelybetween the first and second regular speed ratio node points, thecharacteristic parameters of the four-branch gear system must satisfyfollowing condition:

$\begin{matrix}{\frac{K_{b}\left( {K_{a} + 1} \right)}{K_{b} - K_{a}} \leq \left( \frac{1 + {PR}_{\max}}{1 - {PR}_{\max}} \right)^{2}} & (7)\end{matrix}$

At or in the vicinity of the second speed ratio node point SR2, thetorque of the first electric machine EM1 reverses its direction. As thespeed ratio of the transmission continues to increase, the speed of thefirst electric machine EM1 continues to increase; concurrently, thespeed of the second electric machine EM2 rises from zero and increasesin reverse direction. To avoid excessive internal power circulation whenthe speed ratio of the transmission exceeds far beyond the second speedratio node point SR2, a brake BR may be employed in the transmission tobrake the large sun gear S1, namely the fourth branch of the four-branchgear system, when it is deemed necessary.

3. Reverse Regime

The regime below the natural speed ratio node point SR0 is referred toas the reverse regime. In this regime, the output power split mode isalso applicable. The first clutch CL1 engages and the second clutch CL2disengages. The power is delivered from the ring gear R through two gearstages G1A and G1B, and G4A and G4B to the output shaft system (Output).

To restrict the power split ratio of the electric power path avoidinternal power circulation, pure electric drive mode may be adopted inthe reverse regime. To this end, the electric machine controller (CTRL)controls the second electric machine EM2 to convert electric power fromthe energy storage (BT) into mechanical power, delivering it to theoutput shaft system. The drive torque from the second electric machineEM2 is amplified through the third gear set G3A and G3B and fourth gearset G4A and G4B, and then delivered to the output shaft system.

In fact, pure electric drive operation mode is also applicable inforward speed ratio regime.

Geared Neutral and Parking

Embodiment 2 is capable of providing practical and useful functionsincluding geared neutral and parking. When both the first and secondclutch CL1 and CL2 are disengaged and the first electric machine EM1 isswitched off or at idle state, the transmission is in geared neutral.

Parking can be achieved by engaging the brake BR and both the first andsecond clutches CL1 and CL2. If a third clutch CL3 (FIG. 4) is installedbetween the first and the fourth branches of the four-branch gearsystem, that is between the large sun gear S1 and the small sun gear S2for this particular embodiment, parking can also be achieved by engagingboth the brake BR and the third clutch CL3. In addition, parking can beachieved by conventional parking gear and claws (PBR, not shown)installed on the output shaft system.

Fixed Speed Ratio Operation

Embodiment 2 of the current invention is capable of offering operationsat up to three fixed speed ratios. The fixed speed ratio operations areprovided for special application requirements such as towing andacceleration during hill climbing. The conditions for fixed speed ratiooperations are listed in the following table.

Fixed speed-ratio Status of clutch and brake position CL1 CL2 CL3 BR 1engage engage disengage disengage 2 disengage engage engage disengage 3disengage engage disengage engage

Shifting between adjacent speed-ratio positions is achieved in a smoothand continuous fashion as outlined in previous sections. Thus, there isno power interruption during speed ratio change between fixed speedratios. In addition, at each fixed speed ratio position, both electricmachines (EM1, EM2) can act as motors or generators to provide powerassisting or regenerative braking functions as in a parallel electrichybrid system. This results in enhanced power and performance of thevehicle system.

Since the engagement or disengagement occurs under naturalsynchronization of rotational speed for the involved, clutch orclutches, simple clutches of positive engagement type can be usedinstead of the more complex friction clutches. This eliminates thehydraulic system associated wet friction clutches, and thus effectivelyreduces internal power losses.

Clutch CL1 and clutch CL2 can be integrated to form a combined clutchCL12 (reference to FIG. 10). The combined clutch CL12 is installed onthe counter shaft CTS. The combined clutch CL12 has three operationstatuses: engaging only with gear G1B, engaging only with gear G2B orengaging with both gears G1B and G2B. Similarly, clutch CL3 can beintegrated with brake BR to form a combined clutch-brake CLBR (notshown), installed on the fourth branch, namely on the shaft where thelarge sun gear S1 is connected to. The combined clutch-brake CLBR hastwo operation statuses: engaging with the first branch, namely the smallsun gear S2. Or engaging with a fixed component of the transmission.

Other Operation Status

Embodiment 2 also provides the function for engine start-up. Enginestart-up is accomplished either by one of the two electric machinesindependently or by both electric machines acting collaboratively. Forexample, when the transmission is in geared neutral, the engine can bestarted collaboratively by two electric machines; whereas, when thetransmission is under pure electric drive mode, the engine can bestarted by the first electric machine EM1.

When an energy storage device (BT) is used in conjunction with thedual-mode electro-mechanical variable speed transmission, thetransmission is capable of providing not only a continuous speed ratiovariation but also energy buffering, offering a so-called hybrid driveoperation. Under the hybrid drive operation, power between the twoelectric machines no longer need to be balanced. The electric powergenerated by one electric machine may be more or less than that consumedby the other electric machine. Under such circumstances, the speed rationode point at which one of the electric machines has zero rational speedmay not coincide with the speed ratio at which the other electricmachine has zero torque. The position of speed ratio at which one of theelectric machines has zero torque varies with the power imbalancebetween the two electric machines. However, the position of speed rationode point at which one of the electric machines has zero speed alwaysremains the same regardless the power imbalance between the two electricmachines.

When there is net electric power exchange between the electric powerpath within the transmission and the energy storage device, the electricmachines have to fulfill double duties of both speed ratio regulationand power regulation. Thus, the power ratings of the electric machineshould not be less than the maximum electric power split ratio times therated power at the input shaft of the transmission.

FIG. 5 shows embodiment 2A, a simplified version of embodiment 2. In thesimplified embodiment, the second electric machine EM2 couples directlyto the counter shaft CTS, omitting the third gear set G3A and G3B. Thesecond electric machine EM2, selectively connects either to the secondbranch (ring gear R) through the first single stage gear set G1A and G1Bor to the fourth branch (large sun gear S1) of the four-branch gearsystem through the second single stage gear set G2A and G2B. Inaddition, embodiment 2A removes brake BR and the third clutch CL3.

Embodiment 2A has the same functionalities as embodiment 2 except it nolonger provides the operations at the second and third fixed speedratios.

Embodiment 2 has other variants. For example, FIG. 6 shows anothervariant, embodiment 2B, of embodiment 2. In this embodiment, anothertype of Ravigneaux planetary gear-train was adopted as the four-branchgear system. The Ravigneaux gear-train includes a large ring gear R1 anda small ring gear R2, a set of long planet gears PL, a set of shortplanet gears PS, a planet carrier C and a sun gear S. Each of the longplanet gears PL is in internal meshing engagement with the second ringgear R2 and in external meshing engagement with the sun gear S; each ofthe short planet gears PS is in internal meshing engagement with thefirst ring gear R1 and in external meshing engagement with acorresponding long planet gear PL. The second ring gear R2 constitutesthe first branch (I) of the four-branch gear system, the planet carrierC being the second branch (II), the first ring gear R1 being the thirdbranch (III) and the sun gear S being the fourth branch (IV). Thecharacteristic parameters K_(a) and K_(b) of the four-branch gear systemare related to the teeth numbers of the first and second ring gearsZ_(R1), Z_(R2) and to the teeth number of the sun gear Z_(S)

$\begin{matrix}{{K_{a} = \frac{Z_{S}}{Z_{R\; 1} - Z_{S}}};{K_{b} = \frac{\left( {Z_{R\; 1} + Z_{R\; 2}} \right)Z_{S}}{\left( {Z_{R\; 1} - Z_{S}} \right)Z_{R\; 2}}}} & (8)\end{matrix}$

The structure and connection characteristics of embodiment 2B can beexpressed asR2(EM1)-C(Output,EM2[CL1])-R1(Input)-S(EM2[CL2])

The notations and definition of symbols employed in above expressionfollow the same convention as previously described.

A noticeable characteristic in the layout of embodiment 2B is that thefirst and second electric machines are aligned co-axially withfour-branch gear system. Embodiment 2B has the same functionalities asembodiment 2A.

FIG. 7 shows embodiment 3, the third type embodiment of the currentinvention. Comparing with the second type of embodiment, embodiment 3employs a four-branch differential planetary gear train instead ofRavigneaux planetary gear train. Said four-branch differential planetarygear train includes a first and a second ring gears R1 and R2, a firstand a second sets of planet gears P1 and P2, a planet carrier C and asun gear S. The first ring gear R1 engages with the first set of planetgears P1; the second ring gear R2 engages with the second set of planetgears P2; each and every first planet gear couples with a correspondingsecond planet gear P2, forming a planet pair and having the same axis;the sun gear S engages with the second set of planet gears P2. Thus, thefirst ring gear R1 constitutes the first branch (I) of the four-branchgear system, the second ring gear R2 constitutes the second branch (II),the planet carrier C constitutes the third branch (III) and the sun gearconstitutes the fourth branch (IV). The characteristic parameters offour-branch gear system K_(a), K_(b) are determined by the teeth numbersof the first and second ring gears Z_(R1), Z_(R2), the teeth numbers ofthe first and second planet gears Z_(P1), Z_(P2), and the teeth numberof the sun gear Z_(S)

$\begin{matrix}{{K_{a} = \frac{Z_{S}}{Z_{R\; 2}}};{K_{b} = \frac{Z_{P\; 1}Z_{S}}{Z_{P\; 2}Z_{R\; 1}}}} & (9)\end{matrix}$

The rotor of the first electric machine EM1 couples directly to thefirst ring gear R1, establishing a fixed connection to the first branchof the four-branch gear system with a fixedspeed ratio of 1. The outputshaft system (Output) couples to the second ring gear R2 through gearsets G1A and G1B, and G4A and G4B, establishing a connection to thesecond branch of the four-branch gear system with a speed ratio ofGR1×GR4. Input shaft (Input) couples through a torsion damper (Dmp) tothe planet carrier C, establishing a connection to the third branch ofthe four-branch gear system with a speed ratio of 1. The second electricmachine EM2 selectively couples either to the second ring gear R2through gear set G1A and G1B, establishing a first selective connectionto the second branch with a first speed ratio of GR1, or to the sun gearS through gear set G2A and G2B, establishing a second selectiveconnection to the fourth branch of the four-branch gear system with asecond speed ratio of GR2. Here GR1, GR2 and GR4 are gear teeth ratiosdefined previously. The structure and connection characteristics ofembodiment 3 as shown in FIG. 7 can be expressed as:R1(EM1)-R2(Output,EM2[CL1])-C(Input)-S(EM2[CL2])

The notations and definitions of symbols employed in above expressionare the same as previously described.

Comparing with the simplified embodiment 2A shown in FIG. 5, embodiment3 shown in FIG. 7 has not only employed a different four-branch gearsystem, but also omitted gear G4A, combining it with gear G1B. The poweris transmitted to the output shaft system (Output) through the countershaft with gears G1A, G1B and G4B. Embodiment 3 performs the samefunctions as embodiments 2A and 2B, and thus will not be repeatedherein.

Similarly, embodiment 3 has variants. FIG. 8 shows embodiment 3A, whichis derived from embodiment 3. In this case, the four-branch differentialplanetary gear train includes a first sun gear S1, a second sun gear S2,a first set of planet gears P1, a second set of planet gears P2, aplanet carrier C and a ring gear R. The first sun gear S1 engages withthe first set of planet gears P1. The second sun gear S2 engages withthe second set of planet gears P2. Each and every first planet gear P1couples to a corresponding second planet gear P2, forming a planet pairand having the same axis. The ring gear R engages with the first set ofplanet gears P1. Thus, the ring gear of the four-branch differentialplanetary gear train constitutes the first branch (I); the planetcarrier C constitutes the second branch (II); the first sun gear S1constitutes the third branch (III); and the second sun gear S2constitutes the fourth branch (IV). The characteristic parameters K_(a)and K_(b) of the four-branch gear system are determined by the teethnumbers of the first and second sun gears Z_(S1), Z_(S2), the teethnumbers of the first and second planet gears Z_(P1), Z_(P2), and theteeth number of the ring gear Z_(R):

$\begin{matrix}{{K_{a} = \frac{Z_{P\; 1}Z_{S\; 2}}{{Z_{P\; 2}Z_{S\; 1}} - {Z_{P\; 1}Z_{S\; 2}}}};{K_{b} = {\frac{Z_{P\; 1}Z_{S\; 2}}{{Z_{P\; 2}Z_{S\; 1}} - {Z_{P\; 1}Z_{S\; 2}}}\left( {1 + \frac{Z_{S\; 1}}{Z_{R}}} \right)}}} & (10)\end{matrix}$

The structure and connection characteristics of the transmissionrepresented by embodiment 3A shown in FIG. 8 can be expressed asR(EM1-C(Output,EM2[CL1])-S1(Input)-S2(EM2[CL2])

The notations and definitions of symbols employed in above expressionare the same as previously described.

A common characteristic of the second and third types of embodiments isthat the four-branch gear system is formed by a complete simplethree-branch planetary gear train and an incomplete planetary geartrain. The planet gears in the two planetary gear trains are coupledtogether either in co-axial connection arrangements or in meshingengagements. Said complete three-branch planetary gear train includes aring gear, a set of planet gears, a planet carrier and a sun gear, andhas three co-axial rotating members. Said incomplete planetary gear tramincludes a ring gear, a set of planet gears and a planet carrier, or asun gear, a set of planet gears and a planet carrier. It has twoco-axial rotating members.

FIG. 9 shows embodiment 4 of the current invention, it represents atypical example of the fourth type of embodiment. In contrast to thesecond and third types of embodiments, the four-branch gear systememployed in the fourth type of embodiment is a compound planetary gearsystem formed by two complete three-branch planetary gear trains.

The four-branch planetary gear system of embodiment 4 is comprised of afirst complete simple planetary gear tram and a second complete simpleplanetary gear train. Two members of the first planetary gear traincouple respectively to the corresponding members of the second planetarygear train, forming a so-called four-branch differential planetary gearsystem. Each planetary gear train includes a ring gear (R1 or R2), a setof planet gears (P1 or P2), a planet carrier (C1 or C2) and a sun gear(S1 or S2). The first sun gear S1 of the first planetary gear traincouples to the second sun gear S2 of the second planetary gear train,forming compound member S1S2; the first carrier C1 of the firstplanetary gear train couples to the second planet carrier C2 of thesecond planetary gear train, forming a common planet carrier C1C2. Theseconnections form a four-branch differential planet gear system with thefirst ring gear R1 being the first branch (I), the second ring gear R2being the second branch (II), the common carrier C1C2 being the thirdbranch (III) and the compound first and second sun gears S1S2 being thefourth branch (IV). The characteristic parameters of the four-branchdifferential planetary gear system K_(a), K_(b) are determined by theteeth numbers of the first and second ring gears Z_(R1), Z_(R2) and theteeth numbers of the first and second sun gears Z_(S1), Z_(S2)

$\begin{matrix}{{K_{a} = \frac{Z_{S\; 2}}{Z_{R\; 2}}};{K_{b} = \frac{Z_{S\; 1}}{Z_{R\; 1}}}} & (11)\end{matrix}$

The structure and connecting characteristics of embodiment 4 can beexpressed as:R1(EM1)-R2(Output,EM2[CL1])-C1C2(Input)-S1S2(EM2[CL2])

The symbols and notations employed in above expression are the same aspreviously defined.

Using two three-branch planetary gear trains and coupling two members ofthe first planetary gear train respectively to two dissimilar members ofthe second planetary gear train, or to one similar member and onedissimilar member of the second planetary gear train can also constructa four-branch compound planetary gear system. As an example, FIG. 10shows embodiment 4A where the four-branch compound planetary gear systemis formed by a first simple planetary gear train and a second simpleplanetary gear train. The planet carrier C1 of the first planetary geartrain couples to a similar member, namely the second planet carrier C2of the second planetary gear train, forming a common planet carrierC1C2. The sun gear S1 of the first planetary gear train couples to adissimilar member, the second ring gear R2 of the second planetary geartrain, forming a compound member S1R2. The compound member S1R2constitutes the first branch (I) of the four-branch gear system, thecommon carrier C1C2 constitutes the second branch (II); the first ringgear R1 of the first planetary gear train constitutes the third branch(III) and the second sun gear S2 of the second planetary gear trainconstitutes the fourth branch (IV). The characteristic parameters of thefour-branch gear system K_(a), K_(b) are determined by the teeth numbersof the first and second ring gears Z_(R1), Z_(R2), and the teeth numbersof the first and second sun gears Z_(S1), Z_(S2).

$\begin{matrix}{{K_{a} = \frac{Z_{S\; 1}Z_{S\; 2}}{{Z_{R\; 1}Z_{R\; 2}} - {Z_{S\; 1}Z_{S\; 2}}}};{K_{b} = \frac{\left( {Z_{R\; 1} + Z_{S\; 1}} \right)Z_{S\; 2}}{{Z_{R\; 1}Z_{R\; 2}} - {Z_{S\; 1}Z_{S\; 2}}}}} & (12)\end{matrix}$

The structure and connecting characteristics of embodiment 4A shown inFIG. 10 can be expressed as:S1R2(EM1)-C1C2(Output,EM2[CL12])-R1(Input)-S2(EM2[CL12])

The symbols and notations used in above expression are the same aspreviously defined. CL12 represents an integrated clutch.

In embodiment 4A, the second electric machine EM2 is aligned on the sameaxis as the first electric machine EM1 and the compound planetary gearsystem. The second electric machine EM2 selectively couples to gear G1Bor gear G2B through the integrated clutch CL12, connecting respectivelyto the second branch or the fourth branch of the four branch gearsystem. The integrated clutch CL12 is installed on the counter shaft CTSand has three engagement positions: engaging only with G1B, engagingwith both G1B and G2B, or engaging only with G2B. Usually, theintegrated clutch CL12 is required to provide at least two engagementpositions; that is either engaging with G1B or engaging with G2B.

FIG. 11 shows embodiment 4B of the current invention. In thisembodiment, the four-branch compound planetary gear system is comprisedof a first simple planetary gear train and a second simple planetarygear train. The planet carrier C1 of the first planetary gear traincouples to the ring gear R2 of the second planetary gear train, forminga first compound member C1R2. The sun gear S1 of the first planetarygear train couples to the sun gear S2 of the second planetary geartrain, forming a second compound member S1S2. In this case, the ringgear R1 of the first planetary gear train constitutes the first branch(I) of the tour-branch gear system. The first compound member C1R2constitutes the second branch (II). The planet carrier C2 of the secondplanetary gear train constitutes the third branch (III) and the secondcompound member S1S2 constitutes the fourth branch (IV). Thecharacteristic parameters of the four-branch gear system K_(a) and K_(b)are determined by the teeth numbers of the first and second ring gearsZ_(R1) and Z_(R2), and the teeth numbers of the first and second sungears Z_(S1) and Z_(S2).

$\begin{matrix}{{K_{a} = \frac{Z_{S\; 2}}{Z_{R\; 2}}};{K_{b} = \frac{{Z_{R\; 1}Z_{S\; 2}} + {Z_{R\; 2}Z_{S\; 1}} + {Z_{S\; 1}Z_{S\; 2}}}{Z_{R\; 1}Z_{R2}}}} & (13)\end{matrix}$

The structure and connecting characteristics of embodiment 4B can beexpressed asR1(EM1)-C1R2(Output,EM2[CL1])-C2(Input)-S1S2(EM2[CL2])

The symbols and notations used in above expression are the same aspreviously defined.

Comparing to other embodiments described previously, the first electricmachine EM1 of embodiment 4B couples through gears G5A and G5B to thering gear R1 of the first planetary gear train with a speed ratio ofGR5, establishing a fixed connection to the first branch of thefour-branch gear system. Speed ratio GR5 is calculated as

$\begin{matrix}{{{GR}\; 5} = \frac{Z_{G\; 5B}}{Z_{G\; 5A}}} & (14)\end{matrix}$where Z_(G5A) and Z_(G5B) are teeth numbers of gears G5A and G5B,respectively.

Additionally, in embodiment 4B, the first electric machine EM1, thesecond electric machine EM2 and the four-branch gear system are arrangedon different axes of rotation.

In summary, the basic steps in designing and producing the secondthrough fourth types of embodiments described above include constructinga four-branch gear system and connecting the four branches of the gearsystem to the input shaft, the output shaft system, and the two electricmachines in the following manner: coupling the first electric machine tothe first branch (or the fourth branch) in a fixed connection with afixed speed ratio; selectively coupling the second electric machineeither to the second branch with a first speed ratio (or the thirdbranch) or to the fourth branch (or the first branch) with a secondspeed ratio, coupling the input shaft to the third branch (or the secondbranch) with a fixed speed ratio, and connecting the output shaft systemto second branch (or the third branch) with a fixed speed ratio. Theinput shaft couples to the four-branch gear system through a torsiondamper. The connection of first electric machine to the four-branch gearsystem can be direct or indirect through a single-stage setup gears. Theconnections of the second electric machine and of the output shaftsystem to the four-branch gear system are either through a single-stagegear set or through two-stage gear sets.

The performance characteristics of said four-branch gear system areuniquely defined by its characteristic parameters K_(a) and K_(b). Therelationships between the parameters K_(a) and K_(b), and the teethnumbers of the associated gears are determined by the structure of aspecific four-branch gear system.

For practicality considerations of the four-branch gear system, it isnecessary to impose restrictions on the structure and the characteristicparameters of the four-branch gear system. This is done to ensure thatthe four-branch gear system so constructed is suitable for constructingthe dual-mode electro-mechanical variable speed transmission and iscapable of satisfying all specified functional requirements. Theaforementioned inequality equation (7) set forth the power constraintsfor electric machines from the power matching perspective. It isrecommended that following condition be satisfied when designing andselecting characteristic parameters for a four-branch gear system.

$\begin{matrix}{\frac{K_{b}\left( {K_{a} + 1} \right)}{K_{b} - K_{a}} \leq 2.75} & (15)\end{matrix}$

Additionally, for restricting the rotational speed of electric machines,particularly at high speed regime, it is further recommended that thefollowing relationship holds true.

$\begin{matrix}{\frac{K_{b} + 1}{{GR}\; 5} \leq 3} & \left( {16a} \right)\end{matrix}$

If the first electric machine couples directly to the four-branch gearsystem, that is to say GR5=1, the inequality relationship (16 a)regresses toK_(b)≦2  (16b)

Other four-branch gear systems can be constructed by using planetarygear trains. These four-branch gear systems can be used in turn toderive other useful embodiments that incarnate the current invention.Although these embodiments are not described here, they should beconsidered to be covered under the scope of the current invention.

It should be pointed out that the electric machine referred to in thisdisclosure is a generic term; it refers to both electric motor andelectric generator.

The parts and components required by the aforementioned embodiments canbe readily made by industrial manufacturing means. This warrants thatthe dual-mode electro-mechanic variable speed transmission isobtainable. Said transmission can be operated under two different powersplitting modes, and thus is capable of avoiding internal powercirculation and offering higher power transmission efficiency. Saidtransmission can provide, in a wide speed ratio range, independent andcontinuous output to input speed ratio change and power regulation,extending operation range significantly. The new dual-modeelectro-mechanical variable speed transmission reduces power demand onelectric machines, making the construction of the transmission simple,more compact, and low cost. The transmission is capable of provingcontinuous speed changes from reverse to full stop to forward withoutthe need for vehicle launching device. It significantly improves theoverall fuel efficiency of the vehicle.

The invention claimed is:
 1. A dual-mode electro-mechanical variablespeed transmission comprising: a gear system, an input shaft, an outputshaft system, a first electric machine, a second electric machine and atleast one clutch; said gear system includes at least three componentbranches representing at least three co-axial rotatable components ofthe gear system, respectively; wherein the rotational speeds of any twoof the at least three component branches define the speed for theremaining ones of the component branches, said output shaft systemincludes at least one of an output shaft connected with a drive wheeland a differential connected to the output shaft; the first electricmachine couples in a clutch-less mechanical connection to a firstcomponent branch of the gear system; said input shaft couples directlyor indirectly via a damper device in a rotatable connection to a secondcomponent branch of the gear system; said output shalt system couples ina clutch-less rotatable connection to a third component branch of thegear system; the second electric machine couples selectively via said atleast one clutch to at least the second component branch with a firstspeed ratio or the third component branch with a second speed ratio;whereby the dual-mode variable speed transmission is switchable betweena low speed ratio operation mode and a high speed ratio operation mode.2. The dual-mode electro-mechanical variable speed transmissionaccording to claim 1 wherein switching between the low speed ratiooperation mode and the high speed ratio operation mode is accomplishedthrough said at least one clinch at a mode switching point, wherein atthe mode switching point one of the two electric machines is atessentially zero rotational speed.
 3. The dual-mode electro-mechanicalvariable speed transmission according to claim 1 wherein switchingbetween the low speed ratio operation mode and the high speed ratiooperation mode is accomplished through said at least one clutch at amode switching point, wherein at the mode switching point, one of thetwo electric machines is at essentially zero rotational speed and theother electric machine delivers essentially zero torque.
 4. Thedual-mode electro-mechanical variable speed transmission according toclaim 1 wherein said gear system is a three-branch gear system havingthree component branches, representing three co-axial rotatablecomponents; the rotational speeds of the three component branches arerepresented by a three-branch speed nomograph in which a nomographdistance between the first and second component branches measures K_(a)units in length, a nomograph distance between the second and thirdcomponent branches is 1.0 unit in length; K_(a) is a characteristicparameter of the said three-branch gear system; said third componentbranch of the gear system is selectively coupled via the at least oneclutch to the second electric machine through at least a first pair ofmeshing gears (G1A, G1B), said first pair of meshing gears includes afirst drive gear (G1A) and a first driven gear (G1B) wherein the firstdrive gear (G1A) couples directly to the third component branch of thegear system; said second component branch of the gear system isselectively coupled via the at least one clutch to the second electricmachine through at least a second pair of meshing gears (G2A, G2B), saidsecond pair of meshing gears includes a second drive gear (G2A) and asecond driven gear (G2B), wherein the second drive gear (G2A) couplesdirectly to said second component branch of the gear system; wherein thecharacteristic parameter K_(a), and teeth ratios GR1 and GR2 of thefirst and second pairs of meshing gears satisfy the followingrelationship: ${\frac{{GR}\; 2}{{GR}\; 1} = \frac{K_{a}}{K_{a} + 1}},$where GR1 is the teeth ratio of the first driven gear to the first drivegear and GR2 is the teeth ratio of the second driven gear to the seconddrive gear.
 5. The dual-mode electro-mechanical variable speedtransmission according to claim 1 wherein said dual-mode transmissionincludes a first clutch and a second clutch; said transmission providesat least one fixed output-to-input speed ratio operation by engaging thefirst and second clutches.
 6. The dual-mode electro-mechanical variablespeed transmission according to claim 1 wherein said dual-modetransmission includes at least one counter shaft, said at least oneclutch is installed on said at least one counter shaft; said secondelectric machine couples to said gear system through said at least onecounter shaft.
 7. The dual-mode electro-mechanical variable speedtransmission according to claim 6 wherein the second electric machineconnects to said at least one counter shaft through at least a pair ofmeshing gears (G3A, G3B).
 8. The dual-mode electro-mechanical variablespeed transmission according to claim 1 wherein the output shaft systemcouples to said gear system through at least a pair of meshing gears(G4A, G4B).
 9. The dual-mode electro-mechanical variable speedtransmission according to claim 1 wherein said gear system is athree-branch planetary gear set having three co-axial rotatablecomponent branches that are represented by a sun gear, a planet carrierand a ring gear, respectively; a characteristic parameter of theplanetary gear set K_(a) is represented by the teeth ratio of the ringgear to the sun gear; the first electric machine couples in aclutch-less mechanical connection to the sun gear; the input shaftcouples directly or indirectly via a damping device in a rotatableconnection to the planet carrier; the output shaft system couples in aclutch-less rotatable connection to the ring gear; the second electricmachine couples selectively via said at least one clutch either to thering gear through at least a first pair of meshing gears (G1A, G1B) witha first speed ratio or to the planet carrier through at least a secondpair of meshing gears (G2A, G2B) with a second speed ratio, wherein thefirst speed ratio is different from the second speed ratio; the firstpair of meshing gears includes a first drive gear (G1A) and a firstdriven gear (G1B); the second pair of meshing gears includes a seconddrive gear (G2A) and a second driven gear (G2B); wherein thecharacteristic parameter K_(a) of said planetary gear set satisfies thefollowing relationship${\frac{{GR}\; 2}{{GR}\; 1} = \frac{K_{a}}{K_{a} + 1}},$ where GR1 isthe teeth ratio of the first driven gear to the first drive gear and GR2is the teeth ratio of the second driven gear to the second drive gear.10. A dual-mode electro-mechanical variable speed transmissioncomprising: a gear system, an input shaft, an output shaft system, afirst electric machine, a second electric machine and at least oneclutch; said gear system includes at least three component branchesrepresenting at least three co-axial rotatable components of the gearsystem, respectively; wherein the rotational speeds of an two of the atleast three component branches define the speed for the remaining onesof the component branches; said output shaft system includes at leastone of an output shaft connected with a drive wheel and a differentialconnected to the output shaft; the first electric machine couples in aclutch-less mechanical connection to a first component branch of thegear system; said output shaft system couples in a clutch-less rotatableconnection to a second component branch of the gear system; said inputshaft couples directly or indirectly via a damper device in a rotatableconnection to a third component branch of the gear system; the secondelectric machine couples selectively via said at least one clutch to atleast the second component branch with a first speed ratio or the thirdcomponent branch with a second speed ratio; whereby the dual-modevariable speed transmission is switchable between a low speed ratiooperation mode and a high speed ratio operation mode.
 11. The dual-modeelectro-mechanical variable speed transmission according to claim 10wherein switching between the low speed ratio operation mode and thehigh speed ratio operation mode is accomplished through said at leastone clutch at a mode switching point, wherein at the mode switchingpoint one of the two electric machines is at essentially zero rotationalspeed.
 12. The dual-mode electro-mechanical variable speed transmissionaccording to claim 10 wherein switching between the low speed ratiooperation mode and the high speed ratio operation mode is accomplishedthrough said at least one clutch at a mode switching point, wherein atthe mode switching point, one of the two electric machines is atessentially zero rotational speed and the other electric machinedelivers essentially zero torque.
 13. The dual-mode electro-mechanicalvariable speed transmission according to claim 10 wherein said gearsystem is a three-branch gear system having three component branches,representing three co-axial rotatable components; the rotational speedsof the three component branches are represented by a three-branch speednomograph in which a nomograph distance between the first and secondcomponent branches measures K_(a) units in length, a nomograph distancebetween the second and third component branches is 1.0 unit in length;K_(a) is a characteristic parameter of the said three-branch gearsystem; said second component branch of the gear system is selectivelycoupled via the at least one clutch to the second electric machinethrough at least a first pair of meshing gears (G1A, G1B), said firstpair of meshing gears includes a first drive gear (G1A) and a firstdriven gear (G1B) wherein the first drive gear (G1A) couples directly tothe second component branch of the gear system; said third componentbranch is selectively coupled via the at least one clutch to the secondelectric machine through at least a second pair of meshing gears (G2A,G2B), said second pair of meshing gears includes a second drive gear(G2A) and a second driven gear (G2B), wherein the second drive gear(G2A) couples directly to said third component branch of the gear systemwherein the characteristic parameter K_(a) satisfies the followingrelationship: ${\frac{{GR}\; 2}{{GR}\; 1} = {1 + \frac{1}{K_{a}}}},$where GR1 is the teeth ratio of the first driven gear to the first drivegear and GR2 is the teeth ratio of the second driven gear to the seconddrive gear.
 14. The dual-mode electro-mechanical variable speedtransmission according to claim 10 wherein said dual-mode transmissionincludes a first clutch and a second clutch; said transmission providesat least one fixed output-to-input speed ratio operation by engaging thefirst and second clutches.
 15. The dual-mode electro-mechanical variablespeed transmission according to claim 10 wherein said dual-modetransmission includes at least one counter shaft, said at least oneclutch is installed on said at least one counter shaft; said secondelectric machine couples to said gear system through said at least onecounter shaft.
 16. The dual-mode electro-mechanical variable speedtransmission according to claim 15 wherein the second electric machineconnects to said at least one counter shaft through at least a pair ofmeshing gears (G3A, G3B).
 17. The dual-mode electro-mechanical variablespeed transmission according to claim 10 wherein the output shaft systemcouples to said gear system through at least a pair of meshing gears(G4A, G4B).
 18. The dual-mode electro-mechanical variable speedtransmission according to claim 10 wherein said gear system is athree-branch planetary gear set having three co-axial rotatablecomponent branches that are represented by a sun gear, a planet carrierand a ring gear, respectively; a characteristic parameter of theplanetary gear set K_(a) is represented by the teeth ratio of the ringgear to the sun gear; the first electric machine couples in aclutch-less mechanical connection to the sun gear; the input shaftcouples directly or indirectly via a damping device in a rotatableconnection to the ring gear; the output shaft system couples in aclutch-less rotatable connection to the planet carrier; the secondelectric machine couples selectively via said at least one clutch eitherto the planet carrier through at least a first pair of meshing gears(G1A, G1B) with a first speed ratio or to the nag gear through at leasta second pair of meshing gears (G2A, G2B) with a second speed ratio,wherein the first speed ratio is different from the second speed ratio;the first pair of meshing gears includes a first drive gear (G1A) and afirst driven gear (G1B); the second pair of meshing gears includes asecond drive gear (G2A) and a second driven gear (G2B); wherein thecharacteristic parameter K_(a) of said planetary gear set and teethratios GR1 and GR2 of the first and second pairs of meshing gearssatisfy the following relationship${\frac{{GR}\; 2}{{GR}\; 1} = {1 + \frac{1}{K_{a}}}},$ where GR1 is theteeth ratio of the first driven gear to the first drive gear and GR2 isthe teeth ratio of the second driven gear to the second drive gear. 19.A Dual-mode electro-mechanical variable speed transmission comprising:an input shaft, an output shaft system, a gear system, a first electricmachine, a second electric machine and at least one clutch; said gearsystem includes at least three component branches representing at leastthree co-axial rotatable components of the gear system, respectively;the rotational speeds of any two component branches define the speed ofthe remaining ones of the component branches; said output shaft systemincludes at least one of an output shaft connected with at least a drivewheel and a differential connected to the output shaft; the firstelectric machine couples in a clutch-less mechanical connection to afirst component branch of the gear system; said input shall couplesdirectly or indirectly via a damper device to a second component branchof the gear system; said output shaft system couples in a clutch-lessrotatable connection to a third component branch of the gear system; thesecond electric machine couples selectively via said at least one clutchto the third component branch through at least a first pair of meshinggear with a first speed ratio or to the second component branch throughat least a second pair of meshing gear with a second speed ratio;wherein the first speed ratio is different from the second speed ratio;said dual-mode variable speed transmission is switchable between a lowspeed operation mode and high speed operation mode, the switching isaccomplished via said at least one clutch at a mode shifting point whereat least one of the first electric machine and the second electricmachine is essentially at zero rotational speed; said dual mode variablespeed transmission provides at least a mechanical power path and atleast an electric power path; wherein in said high speed operation modeand when essentially zero power is received from or delivered to energystorage system, the power that passes through the electric power path isalways less than the power transmitted through the transmission from theinput shaft to the output shaft system, and no internal powercirculation occurs.